Hi there!
"The sum of n and 6" means n plus 6, (n+6) or (6+n).
We're looking for the answer that does NOT mean the sum of n and 6.
"6 + n" means the sum of 6 and n, which is the same as the sum of n and 6.
"n + 6" means the sum of n and 6, which is the same as the sum of 6 and n.
"6n" means 6 multiplied by n, which is the same as n multiplied by 6. However, this does NOT mean the sum.
6n is your answer.
I hope this helps!
